New retroreflective material could be used in nighttime color-changing road signs

New retroreflective material could be used in nighttime color-changing road signs
An image series shows how a new retroreflective material can be used to make a color-changing speed limit sign. Boxes A-F show how the sign changes color, from the perspective of drivers on the road, as they pass by. Credit: Fan et al., Sci. Adv. 2019; 5 : eaaw8755. CC BY-NC

A thin film that reflects light in intriguing ways could be used to make road signs that shine brightly and change color at night, according to a study that will be published on Aug. 9 in Science Advances.

The technology could help call attention to important traffic information when it's dark, with potential benefits for both drivers and pedestrians, researchers say.

The film consists of polymer microspheres laid down on the sticky side of a transparent tape. The material's physical structure leads to an interesting phenomenon: When white light shines on the film at night, some observers will see a single, stable color reflected back, while others will see changing colors. It all depends on the angle of observation and whether the is moving.

The research was led by Limin Wu, Ph.D., at Fudan University in China, whose group developed the material. Experts on optics at the University at Buffalo made significant contributions to the work, providing insight into potential applications for the film, such as employing it in nighttime road signs.

"You can use this material to make smart traffic signs," says Qiaoqiang Gan, Ph.D., an associate professor of electrical engineering in the UB School of Engineering and Applied Sciences and a co-first author of the new study. "If a person is listening to loud music or isn't paying attention while they're walking or driving, a color-changing sign can help to better alert them to the traffic situation."

Testing color-changing road signs at night

In one set of experiments, researchers created a speed limit sign with letters and numbers made from the new film. The scientists placed a white light nearby to illuminate the sign, and when a fast-moving car drove past, the color of the characters on the sign appeared to flicker from the perspective of the driver as the driver's viewing angle changed.

In other tests, the team applied the new material to a series of markers lining the side of a road, denoting the boundary of the driving lane. As a car approached, the markers lit up in bright colors, reflecting from the vehicle's headlights.

From the driver's perspective, the markers' color remained stable. But to a pedestrian standing at the side of the , the color of the markers appeared to flicker as the car and its headlights sped past.

"If the car goes faster, the pedestrian will see the more quickly, so the tells you a lot about what is going on," says co-author Haomin Song, Ph.D., UB assistant professor of research in electrical engineering.


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More information: "Iridescence-controlled and flexibly tunable retroreflective structural color film for smart displays," Science Advances (2019). DOI: 10.1126/sciadv.aaw8755
Journal information: Science Advances

Citation: New retroreflective material could be used in nighttime color-changing road signs (2019, August 9) retrieved 21 August 2019 from https://phys.org/news/2019-08-retroreflective-material-nighttime-color-changing-road.html
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Aug 10, 2019
Way more valuable for drive camera readable signs if you ask me. A road worker can plop down a sign for speed limit change for construction zone, and drive camera can detect and read it based on predictable color changes, parse it into data, gps tag it and alert gps of all other vehicles, creating instant gps speed limit change from one temporary sign.

Aug 10, 2019
Not a bad idea, @luke. Hard to claim you "dint see it" when it's flashing colors at you.

Aug 11, 2019
Why not just use LEDs for different colors and make the electronics control system just keep changing the colors of the LED image displayed exactly like I have seen done in some types of Christmas light decorations? What I am really trying to ask here is; why would it be better making the display work via reflection of light (as they propose to do here) than making it work only via direct emission of light?

Aug 11, 2019
drive camera can detect and read it based on predictable color changes


It doesn't work reliably because there are multiple cars on the road shining their headlights towards the sign, and the road isn't perfectly level or even, so the headlights bounce around and the colors flicker erratically.

Then there's also adaptive headlights (led matrix) that try to reduce glare by detecting other cars' headlights and pointing their lights away from them, which may also happen with a retro-reflective road sign.

Aug 11, 2019
why would it be better making the display work via reflection of light


It doesn't require any power to operate and it's maintenance-free since there's no mechanism or electronics inside.

Aug 11, 2019
why would it be better making the display work via reflection of light


It doesn't require any power to operate
False. A light is used to light it up. You can see that in the above photo of it. In fact, because of selective absorption, doing it this way probably requires MORE power.
and it's maintenance-free since there's no mechanism or electronics inside.
Which is a pretty trivial advantage at best (if it exists at all) because electronic for LED displays generally require extremely little maintenance as modern electronics are relatively reliable. But it isn't always maintenance-free; what if the light that lights it fails? Or what if algae or moss or dirt comes to cover the display so to block the reflected light? etc.

Aug 11, 2019
drive camera can detect and read it based on predictable color changes


It doesn't work reliably because there are multiple cars on the road shining their headlights towards the sign,
Look at the photo of it above that shows a permanent night-light shining on it especially for that purpose. So it does 'work' but that's not what I have issue with.

Aug 11, 2019
the colors flicker erratically
The flickering lights will register in the visual cortex which is tuned by our evolution to spot such changes, which it registers as motion. It will have a greater impact than just the sign with no such treatment, which is an advantage if you want to know what the speed limit is.

Aug 11, 2019
@humy:

The figure shown with the article is bizarre for several reasons. First, the sign appears to be situated parallel to the direction of travel. No road signs are erected like that. Hence, if set up like in the figure, illumination other than car headlights would be needed - but they aren't set up like that. Second, it is doubtful you'll find many roads with an unprotected bike lane and pedestrian walkway that close to a high speed road. US road construction standards would forbid it. The figure doesn't portray reality.

The article is not behind a paywall. Another, more realistic figure with a sign surface set up at a right angle to travel direction - like all signs are - is shown with no extra illumination.

I worked for a time for a state highway department. With budget shortfalls always, they wouldn't want to use extra lights unless necessary or in high traffic areas. Passive systems are what is desired. Your LED concept wouldn't fly except in limited areas like a beltway.

Aug 11, 2019
False. A light is used to light it up.


IT (the speed sign) doesn't require any power to operate. Whether you also place a light in front of it is another question, because it's not strictly necessary. The car's headlights and the streetlights will do the job.

The flickering lights will register in the visual cortex which is tuned by our evolution to spot such changes, which it registers as motion.


Yes, but the answer was for the idea that a computer spots and reads the sign by "predictable color changes", which doesn't work because the flickering depends on random factors - it's not so predictable, so you can't cheat the image recognition problem that way.

Aug 11, 2019
@humy:

The figure shown with the article is bizarre for several reasons. First, the sign appears to be situated parallel to the direction of travel. No road signs are erected like that. Hence, if set up like in the figure, illumination other than car headlights would be needed - but they aren't set up like that. ...The figure doesn't portray reality.

... With budget shortfalls always, they wouldn't want to use extra lights unless necessary or in high traffic areas. Passive systems are what is desired.
True. What you say here makes sense to me.

Aug 11, 2019
Which is a pretty trivial advantage at best (if it exists at all) because electronic for LED displays generally require extremely little maintenance as modern electronics are relatively reliable.


Passive paint vs. LEDs in terms of maintenance is not even a comparison. LED displays require power cabling, or batteries and solar panels, all of which requires at least periodic inspection and eventually replacement. There's hundreds of times more things to go wrong than simply having some paint over a piece of galvanized steel stamping.

Even for cleaning, you just drive around with a tank truck and spray them down with water, or if it's really bad you replace the entire sign, whereas with LED lights you need someone to climb up the pole (or take the lights down) to check that the cabling hasn't cracked, there's no water leakage or fire ants nested inside the electronics... etc. etc. which takes ages and costs a ton per unit.


Aug 11, 2019
And speaking of retro-reflective paints; some yokel here has sprayed the highway exit signs with clear lacquer, changing or spraying over some of the letters on the signs.

It doesn't show up at daytime, but at night the lacquer doesn't reflect and the "amendments" become visible, so the signs say something different. The local authority's basically gone "meh" and left them like that.

Aug 12, 2019
I'd love to play with it, see what the rules are. Does spectrum depend on orientation of the material? Haven't read the paper yet, but I'll bet it has some orderly rules you could leverage in cool ways for computer vision. Passive materials are just generally cool, though IMHO roadside electronics are under leveraged at this point, with little computers and solar panels getting cheaper all the time. At present, a raspberry pi costs as much as 3 traffic cones.

Aug 12, 2019
The flickering lights will register in the visual cortex which is tuned by our evolution to spot such changes, which it registers as motion.


Yes, but the answer was for the idea that a computer spots and reads the sign by "predictable color changes", which doesn't work because the flickering depends on random factors - it's not so predictable, so you can't cheat the image recognition problem that way.
The driver is responsible for the car. Not a computer. Better they see the sign than not.

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